mirror of https://github.com/texmacs/doc.git
117 lines
4.3 KiB
Tcl
117 lines
4.3 KiB
Tcl
<TeXmacs|1.0.3.7>
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<style|tmdoc>
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<\body>
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<tmdoc-title|Evaluation control>
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The <menu|Source|Evaluation> menu contains several primitives to control
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the way expressions in the style-sheet language are evaluated. The most
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frequent use of these primitives is when you want to write a ``meta-macro''
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like <markup|new-theorem> which is used for defining or computing on other
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macros. For instance:
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<\tm-fragment>
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<inactive*|<assign|new-theorem|<macro|name|text|<quasi|<assign|<unquote|name>|<macro|body|<surround|<no-indent><strong|<unquote|<arg|text>>.
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>|<right-flush>|<arg|body>>>>>>>>
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</tm-fragment>
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When calling <inactive*|<new-theorem|theorem|Theorem>> in this example, we
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first evaluate all <markup|unquote> instructions inside the <markup|quasi>
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primitive, which yields the expression
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<\tm-fragment>
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<inactive*|<assign|theorem|<macro|body|<surround|<no-indent><strong|Theorem.
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>|<right-flush>|<arg|body>>>>>
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</tm-fragment>
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Next, this expression is evaluated, thereby defining a macro
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<markup|theorem>.
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It should be noticed that the <TeXmacs> conventions for evaluation are
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slightly different then those from conventional functional languages like
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<name|Scheme>. The subtle differences are motivated by our objective to
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make it as easy as possible for the user to write macros for typesetting
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purposes.
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For instance, when <TeXmacs> calls a macro
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<inactive*|<macro|<active*|x<rsub|1>>|<active*|<with|mode|math|\<cdots\>>>|<active*|x<rsub|n>>|body>>
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with arguments <verbatim|y<rsub|1>> until <verbatim|y<rsub|n>>, the
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argument variables <src-arg|x<rsub|1>> until <src-arg|x<rsub|n>> are bound
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to the unevaluated expressions <verbatim|y<rsub|1>> until
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<verbatim|y<rsub|n>>, and the body is evaluated with these bindings. The
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evaluation of <verbatim|y<rsub|i>> takes place each time we request for the
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argument <no-break><src-arg|x<rsub|i>>. In particular, when applying the
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macro <inactive*|<macro|x|<arg|x> and again <arg|x>>> to an expression
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<verbatim|y>, the expression <no-break><verbatim|y> is evaluated twice.
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In <name|Scheme>, the bodies of <name|Scheme> macros are evaluated twice,
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whereas the arguments of functions are evaluated. On the other hand, when
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retrieving a variable (whether it is an argument or an environment
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variable), the value is not evaluated. Consequently, a <TeXmacs> macro
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<\tm-fragment>
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<inactive*|<assign|foo|<macro|x|<blah|<arg|x>|<arg|x>>>>>
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</tm-fragment>
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would correspond to a <name|Scheme> macro
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<\scheme-fragment>
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(define-macro (foo x)
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\ \ `(let ((x (lambda () ,x)))
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\ \ \ \ \ (blah (x) (x)))
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</scheme-fragment>
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Conversely, the <name|Scheme> macro and function
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<\scheme-fragment>
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(define-macro (foo x) (blah x x))
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(define (fun x) (blah x x))
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</scheme-fragment>
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admit the following analogues in <TeXmacs>:
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<\tm-fragment>
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<\inactive*>
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<assign|foo|<macro|x|<eval|<blah|<quote-arg|x>|<quote-arg|x>>>>>
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</inactive*>
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<\inactive*>
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<assign|fun|<macro|x|<with|x*|<arg|x>|<blah|<quote-value|x*>|<quote-value|x*>>>>>
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</inactive*>
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</tm-fragment>
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Here the primitives <markup|quote-arg> and <markup|quote-value> are used to
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retrieve the value of an argument <abbr|resp.> an environment variable. The
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<TeXmacs> primitives <markup|eval>, <markup|quote>, <markup|quasiquote> and
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<markup|unquote> behave in the same way as their <name|Scheme> analogues.
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The <markup|quasi> primitive is a shortcut for quasi-quotation followed by
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evaluation.
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<tmdoc-copyright|1998--2004|Joris van der Hoeven>
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<tmdoc-license|Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation; with no
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Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled "GNU Free
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Documentation License".>
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</body>
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<\initial>
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<\collection>
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<associate|page-bot|30mm>
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<associate|page-even|30mm>
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<associate|page-odd|30mm>
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<associate|page-reduce-bot|15mm>
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<associate|page-reduce-left|25mm>
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<associate|page-reduce-right|25mm>
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<associate|page-reduce-top|15mm>
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<associate|page-right|30mm>
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<associate|page-top|30mm>
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<associate|par-width|150mm>
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</collection>
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</initial> |